The sector of industrial marking using laser is in considerable expansion subsequent to the possibility of marking metallic and non-metallic materials with techniques such as surface or deep engraving of the material or through colour change of the material. In this sector solid state lasers excited by laser diodes (Diode-Pumped Solid-State, or DPSS) are conventionally used, with average powers generally below 100 W and operating with light pulse repetition obtained through the “Q-switching” technique. The laser beam, produced by diodes, is then sent, through suitable collimation optics, to the work surface. This transfer can take place in two ways: by moving the laser beam on a fixed sample using a system of galvanometric mirrors, or by moving the sample using a system of axes x-y-z on a fixed laser beam. The DPSS laser source is typically constituted by discrete optical components such as: mirrors, crystals, lenses and prisms. A laser diode for excitation, also called power pump diode, excites a suitably doped optical crystal. When population inversion takes place inside the crystal, a coherent and monochromatic electromagnetic radiation is generated at the wavelength corresponding to the emission transition of the doped crystal. This radiation is amplified inside the resonant laser cavity delimited by two mirrors: a mirror through which the pump beam is sent to the crystal, known as High Reflection Mirror (HR) and an Output Coupler (OC) mirror, thus giving rise to the laser radiation. Deconvolution of the emission spectrum of the crystal with the reflection and pass bands of said mirrors produces a highly monochromatic radiation (<0.1 nm). An acoustic-optical modulator, i.e. based on sound-light interaction, positioned inside the cavity produces a pulsed signal giving rise to Q-Switching. However, this solution presents some problems.
It is known that DPSS lasers are affected by the problem of thermal lens that causes a variation in the quality of the laser mode as a function of the intensity of the pump diode with which the crystal of the active material is irradiated. Consequently, the quality of the output beam, and therefore the quality of the industrial marking, depends on the output power. Moreover, the quality of the laser beam (known as Beam Quality Parameter, M2) depends on the optical length of the resonator, defined as the distance between the HR and OC mirrors. That is, by varying the optical length, the quality of the beam changes. The time duration of the pulses also depends on this length. It is possible to define the time duration of the laser pulse by varying the aforesaid length. By increasing this length the time duration of the pulses increases. This variation is not random, but is dependent on the stability parameters of the laser, which in turn are dependent on the optical properties of the OC and HR mirrors and on the length of the thermal lens. However, when obtaining a stable laser cavity, such as to guarantee pulses of long duration, the problem of optical stability (optical alignment) of the cavity must be tackled. We can conclude that with DPSS technology it is not possible to obtain a laser source with a long pulse duration and simultaneously high mode quality. Moreover, the reliability of the laser source is limited as it is constituted by discrete elements and the cost of the whole system is relatively high and cannot be significantly decreased, as the cost of the discrete components cannot be further reduced, given that they cannot be mass produced.
For industrial machining operations, the DPSS laser source must be kept close to the work surface, in a single and relatively voluminous system, as transfer of the beam from the laser to the surface occurs through propagation in the free space. An alternative technology to the above relates to the use of a pulsed fibre laser. In a particularly common configuration, the laser is constituted by an oscillator that pumps a power amplifier, both made completely in optical fibre. In this architecture, the laser wavelength is not generated by pumping an optical crystal, as is the case in DPSS technology, but an optical fibre, known as active fibre, suitably doped with rare earths. There are two types of fibre laser architectures that allow pulsed laser beams to be produced. The first uses a low power seeder diode (a few tens of mW) whose signal, electronically pulsed, must be amplified several times to reach a sufficient power value. In the second type the laser beam, again emerging from a lower power diode, is pulsed using a Q-Switch, connected with the fibre. The beam output from this chain is conveyed, through a further fibre, to the work surface, guaranteeing remote positioning of the laser beam. Compared with the DPSS laser, the fibre laser has undoubted advantages. The quality of the beam and consequently of the marking does not depend on the output power and on the repetition rate, i.e. the fibre laser is not dependent on the effect of the thermal lens. Unlike DPSS lasers, the high quality of the laser beam (M2≈1) is not dependent on the laser power and is established by the single mode of the fibre.
Optical fibre components already available and with relatively low costs are used, as these are used in the telecommunications sector and are readily available. The source is more reliable, as the laser beam always propagates in optical fibre and no discrete optics are involved, as instead is the case in DPSS lasers. Finally, the source can be positioned remotely with respect to the work surface, as the beam is conveyed towards it directly from the optical fibre, while in DPSS lasers the beam is propagated to the work surface in the air. However, it must be stated that this latter solution also has undeniable disadvantages or problems. The intensity and the shape of the laser pulse emerging from a fibre laser is greatly influenced by non-linear phenomena such as: Scattering, i.e. Stimulated Brillouin Scattering (SBS), Photodarkening, Amplified Spontaneous Emission (ASE), which occur in the optical fibre and, principally, in the oscillator, phenomena that are completely absent in DPSS lasers as the laser radiation is produced in a crystal. In particular, phenomena of scattering, such as Stimulated Brillouin Scattering, compete largely with the laser efficiency giving rise to pre- and post-pulses that are amplified in the subsequent amplification chain reducing the efficiency of the laser and producing signals that can counterpropagate in the chain of the fibre laser colliding with and damaging sensitive elements such as the pump diodes. It is not possible to compensate this loss of efficiency by increasing the length of the fibre, as the intensity of Stimulated Brillouin Scattering depends on this parameter. Therefore, the longer the active fibre is, the more probable all non linear phenomena are, as for example in fibre laser architecture composed of a plurality of amplification stages.
Moreover, fibre laser architecture which involves the use of the Q-Switch is complicated by the need to launch the signal emerging from the fibre in the Q-Switch and subsequently receive it in this fibre. This type of architecture has a complex optical design, the objective of which is to guarantee maximum launching and collection efficiency in the single-mode fibre. This complexity is inexistent in a DPSS cavity as the Q-Switch must be proportioned only to the dimension of the laser mode. The output light is not linearly polarized, as is the case in some DPSS lasers, a fact that prevents possible and effective frequency duplication of the output beam, particularly useful in the case of micromachining operations, such as the formation of solar cells. Moreover, this radiation has a spectral width greater than one nanometer (FWHM Full Width at Half Maximum) not suitable for harmonic conversion, which is instead possible with laser radiation emitted from a DPSS source.